We plan to localize and determine levels of carbonic anhydrase (CA) in osteoclasts of chick bone. Metatarsi of 3-5 week old chicks contain large numbers of osteoclasts and relatively few blood cells which also contain CA. Osteoclasts will be collected, counted and assayed for CA. A fixation procedure has been developed which will permit us to localize CA at the electron microscope level. High specific activity H3-acetazolamide which specifically binds to CA is used as the tracer. We also plan to use a ferritin-labeled antibody procedure to locate the enzyme. We have already developed a rabbit anti-chicken CA antibody which we will use in conjunction with ferritin-labeled goat anti-rabbit antibody. To effect this procedure we will explore fixation techniques which minimally interfere with the antigen-antibody reaction in the tissue. If we know the level of CA in osteoclasts and its intracellular localization we should have a better comprehension of the mechanism of osteoclastic function.